1. Field of the Invention
This invention relates generally to hydrostatic motors and, more particularly, to a hydrostatic motor unit having at least two hydraulic motors that can be connected in parallel, at least one of which has a variable absorption volume, in which the hydraulic motors are coupled to each other on the output side with the interposition of a transmission.
2. Description of the Currently Available Technology
A known motor unit used as the central drive system of a work machine is described in DE 42 35 710 A1. This motor unit consists of a high-speed axial piston motor that has a variable absorption volume and, located axially parallel to it, an internally pressurized radial piston motor that has a constant absorption volume and is used as a low-speed motor. The output shaft of the axial piston motor is provided with a gear wheel which is engaged with gearing that is located on the outside periphery of the rotationally mounted housing of the radial piston motor. The difference in speed between the high-speed motor and the low-speed motor is equalized by a gear ratio reduction in the transmission formed by the gear wheel and the gearing.
When connected hydraulically in parallel, the two hydraulic motors--as a function of their individual absorption volume--each generates an output moment. The two output moments are measured together (summed) to define the total output moment. The radial piston motor can be deactivated by isolating it from the hydraulic circuit and by pressurizing the pistons toward their inner dead center position. The hydraulic fluid released (freed) is thereby available to the axial piston motor. The axial piston motor is set to the minimum absorption volume immediately before the radial piston motor is shut off. When the radial piston motor is shut off, the axial piston motor is very quickly returned to maximum absorption volume, so that it can absorb the additional hydraulic fluid from the radial piston motor without any shifting jolts. This process corresponds to a synchronization. By the subsequent reduction of the absorption volume of the axial piston motor--with a reduced total output moment of the motor unit--the output speed of the motor unit can be further increased compared to the hydraulic parallel operation of the two hydraulic motors, and the possible range of speeds at which the motor unit can be used can be increased.
On account of its large size and the cost and complexity of its construction, the motor unit described above is unsuitable for use in restricted spaces, such as wheel drives. In most cases, such applications require high-speed variable axial piston motors, downstream of which a step-down transmission is connected.
The known art also includes the use of low-speed radial piston motors as wheel drives (direct drive). Radial piston motors, however, do not have a variable absorption volume in a multiple-stroke configuration, which significantly restricts the range of potential applications. In a single-stroke configuration, of course, the absorption volume can be adjusted, but in that case such drive mechanisms represent high-speed motors which require a subsequent reduction in the gear ratio if low output speeds and a high output moment are required.
Therefore, it is an object of this invention to provide a hydrostatic motor unit which is simple, compact and can be used over a wide range of applications.